Pads exposed from solder resists are provided for soldering with surface mount devices, such as ball grid array (BGA), quad flat package (QFP) and so on. In general, the pads made of copper, gold, or silver etc. are not easily to bind with solder immediately. Therefore, the pads are usually precoated with the solder for improving solder wettability and self-alignment effect, while bumps of the BGA or a flip chip are soldered with the pads. In addition, the precoated solder generally forms with certain heights and shapes on the pads, such spherical shapes for soldering with the bumps of the BGA or the flip chip easily. Thus, the solder precoating procedure is considered as an essential procedure for the surface mounting technology.
A conventional method for solder precoating applies a stencil printing method. The stencil printing method firstly applies a mask with hollowed patterns to a circuit board and the hollowed patterns of the mask are designed for corresponding to pads of the circuit board. Then, mask is overlaid to the circuit board and the hollowed patterns of the mask are filled with solder paste by a squeegee. Next, remove the mask and apply a reflow process to the circuit board so as to make the solder paste formed into solder bumps for soldering with the surface mount devices.
In addition, the hollowed patterns of mask are hard to achieve fine pitch demands if have solder height requirement, such as bump pitch <150 um, because limitations in thickness of the mask and means to form the hollowed patterns. In other words, the thickness of the mask is thicker, the yield of the solder bumps are poorer, namely fail in controlling the quantity of the solder paste. Therefore, due to the limitations, the solder bumps having a height of 20 um or 20 um above are in great demand, due to some devices need more solder while soldering.